Many computer-based information systems utilize communication configurations known as Local Area Networks (LANs). Data communication in LAN systems may be implemented in various ways, including using light-wave communication. LAN light-wave communication systems typically utilize light emitting diodes (LEDs) or semiconductor lasers as light sources for a fiber-optic communications network, from which optical receivers receive the light-wave information carrying the system data.
In LAN light-wave communication systems, the speed and quality of data transfer between sending and receiving units may be limited by characteristics, such as electrical response time, of the system components. For example, if the light transmitting element is an LED, the LED has a certain response time corresponding to a time between when a data signal is received by the LED and when a corresponding light-wave signal is transmitted by the LED.
An example prior art electro-optical transmitter is shown in FIG. 1 and includes a data signal source (not shown) which provides a data signal on line 12. The signal on line 12 is coupled to LED 16 via resistor 14, which limits the current through LED 16. Assuming resistor 14 is 17 .OMEGA. and LED 16 is a Hewlett Packard.TM. QFBR1590 660 nm LED, the response time of LED 16 is about 50 ns, limiting the signal transmission rate to about 10 MBd.
FIG. 2 shows a prior art circuit with improved response time of the transmitting LED. The prior art shown in FIG. 2 can achieve a rise time on LED 128 of 25.2 ns, better than twice as fast as the response time of LED 16 in FIG. 1. The prior art circuit of FIG. 2 is optimal for use with a type of high speed infrared fiber optic LEDs, often referred to as edge-emitter LEDs.
Edge emitter LEDs are characterized by a low junction capacitance. The primary cause of turn on and turn off delay in edge emitter LEDs is minority carrier lifetime. To minimize delay caused by minority carrier lifetime, resistors 124 and 126 are used to minimize minority carrier build up. Capacitor 122 provides some compensation for delays due to the junction capacitance, since the junction capacitance is small, but the circuit is unable to compensate for large junction capacitances such as in LED 16 of FIG. 1. Resistors 120 and 124 limit the DC current through LED 128.
What is desired, is a LAN electro-optical interface system with improved data transfer speed, in spite of component response time limitations.